Robust Inorganic Daytime Radiative Cooling Coating Based on a Phosphate Geopolymer

Daytime radiative cooling can spontaneously cool an object by reflecting sunlight and radiating heat in the form of infrared rays. Current daytime radiative cooling designs, including photonic structures and organic polymer-dielectric systems, are prone to age and fail under harsh conditions including high temperature, mechanical wear, and/or space irradiation. Here, an all-inorganic phosphoric acid-based geopolymer (PGEO) paint was developed and showed robust radiative cooling performance. This versatile suspension paint can be applied directly to diverse surfaces through scalable techniques such as spray coating and brushing. This inorganic coating possesses a high average hemispherical infrared emissivity >0.95 and reflects nearly 90% of solar irradiance. We attributed this excellent spectral selectivity of the PGEO coating to its unique inorganic geopolymer network (-Si-O-Al-O-P-O-), which settled the vibration intensity in a suitable range (0.2 < k < 1) and enabled multimode vibration. Moreover, this inorganic coating exhibits good comprehensive performance in terms of heat endurance, mechanical strength, and resistance to intense proton radiation, showing its promising applications in spacecraft, buildings, and communication base stations.